LED lighting is starting to become a mainstream choice for low energy lighting. In order for LED technology to fully establish its credentials in this field, and to fit in with current expectations of users, it is highly advantageous for LED-based lighting systems to provide a means for dimming the light intensity emitted, in response to an ‘instruction’, or signal, provided by a dimming means. Such a means can take one of several forms, depending upon the age, purpose and architecture of the lighting system. One of the most common forms, is termed a phase-modulation approach, and is applied to the incoming AC voltage. Such a process involves the interruption of the alternating input voltage over part of its positive and negative cycles and is often referred to as ‘phase cutting’. The degree of interruption is expressed as a ‘cut angle’—this being the phase angle of each half-cycle during which the input voltage is zero. Such a process essentially modulates the RMS voltage available to a ballast, or driver, which in turn, modulates the DC power provided to a light source. The ballast therefore provides a transfer function between the degree of phase cutting provided by a dimmer switch or slider, and the luminous output of the light source.
Dimmer controls that use the phase cutting approach, have been in use for many years, and were designed originally for use with tungsten light bulbs. It is convenient therefore for a ballast associated with a replacement LED bulb to be able to function with such controls.
In LED lighting, the light source, being a semiconductor junction device, is sensitive to electrical impulses up to very high frequencies. More specifically, the light output from an LED is directly related to the electrical current flowing through it. Therefore, the electrical impulse to which an LED is sensitive, and in response to which it's light output will change, is any change in LED current. So, if the LED current fluctuates, so does the light output. This is the basic origin of LED flicker. In addition to this, any fluctuations in the current supplied to the LED can introduce additional heating within the device, thereby reducing its lifetime.
For LED lighting—from domestic lighting and industrial lighting, through to street-lighting and signage—there are two major performance shortfalls that currently stand in the way of widescale technology adoption. These are lifetime and flicker. Given that both of these are related in part, to LED current ripple, it is desirable that lighting ballasts are developed that provide sufficiently low levels of ripple. However, as explained later, flicker can also be produced by low frequency modulation, which is frequently applied to LEDs as a means of dimming.
In the general case of dimmable LED lighting, there can be two major sources of photometric flicker. The first of these is a current or voltage tone at the second harmonic of the AC line frequency. The presence of this second harmonic (at twice the AC line frequency) arises as a result of full-wave rectification of the incoming mains—normally performed using a diode bridge. This fluctuation—normally in the frequency range 100 to 120 Hz—will, if un-filtered, give rise to flicker in the LED load. The most common method for filtering this unwanted output from an LED lighting ballast based on an isolated topology, such as a Flyback Converter, is to place a large-value smoothing capacitor at the output of the ballast, where the said capacitor sits in parallel with the LED load. A voltage ripple is produced across this capacitor, in response to the fact that full-wave rectified current is supplied to it by the ballast, whilst a DC current is being extracted by the LED load. With all other things being equal, the higher the value of this smoothing capacitor, the lower the voltage ripple produced across it at the second harmonic frequency, and therefore, the lower the current ripple in the LED load produced thereby. When using this approach, however, practical limits exist on the size of the capacitance. For instance, if an LED lighting ballast is used in conjunction with a phase-cut dimmer employing a leading edge phase cut, then high values of capacitance at the output of the ballast can give rise to instabilities in the dimmer and/or ballast.
The second source of flicker that can occur in the general case of LED dimming is the use of Pulse Width Modulation (PWM) for the purposes of dimming. This involves switching the LED current on and off, or on and nearly off, thereby sampling the on-state current, through what is essentially a time-domain gating process. Such a process generally introduces a flickering mechanism, whereby flicker occurs at the frequency of the PWM waveform. In extremis, when the off-state current is zero, the flicker has a modulation depth (often referred to as ‘flicker percentage’) of 100%. Research has shown that the sensitivity of an observer, to such flicker is strongly related to the frequency of the flicker and therefore to the frequency of the PWM. Depending upon various factors, including the presence of other light sources, it has been determined that a significant proportion of the population are sensitive to stroboscopic effects arising as a result of LED light flicker at frequencies up to at least 1.25 KHz, and frequently up to 3 KHz.
In view of the preceding, for the purpose of minimising low frequency photometric flicker, it is advantageous to reduce this said ripple current, whilst ensuring that any PWM applied to the light for the purposes of dimming is at a frequency above at least 1.25 KHz, and preferably above 3 KHz.
The process by which ripple current is produced in a phase-dimmable LED lighting ballast can be illustrated by reference to FIGS. 1 and 2. FIG. 1 shows, in schematic form, a dimmable LED lighting scheme, wherein an incoming AC mains voltage Vac is phase-cut by a phase cutting dimmer (1). After passing through a diode bridge (2) and a Flyback converter, or similar power converter circuit (3) operating in constant current mode by reference to the LED load current (ILED) a full-wave rectified current is injected into the parallel combination of output capacitor (4) and LED load (5). FIG. 2 shows the time domain waveforms of the current, i(t) into the said parallel combination and the voltage, v(t) across it. The voltage waveform is applied across the LED load, which in turn typically comprises a string of series connected LEDs. It is relatively simple to appreciate that, in view of the low differential impedance of LEDs (i.e. the rate of change of voltage with current) any such voltage waveform, containing as it does, appreciable peak-to-peak ripple, will give rise to significant current ripple in the/each LED string. Such current ripple will in-turn, give rise to flicker in the light emitted by the LEDs.
Referring to FIGS. 1 and 2, as the cut angle, ϕ increases within the first quadrant (from 0 to 90°) the amount of charge injected into the capacitor (4) by the power converter circuit, during each cycle of the mains, reduces. Consequently, if the current taken by the load (5) was to remain constant during the first stages of dimming (from ϕ=0 to some phase angle within the first quadrant) then the peak-to-peak voltage ripple across the capacitor (4)—shown by the bold line in FIG. 2—would increase. This in turn, in the absence of a ripple suppression mechanism, would give rise to increased current ripple in the LED load and therefore, increased flicker.
This presents a significant challenge in the case of ballasts that exhibit appreciable current ripple in the un-cut (ϕ=0) state. Any increase in this ripple during low angle cutting will exacerbate flicker in the load. One manifestation of this would arise in the case of a conventional ballast being used in conjunction with a phase-cutting dimmer which has an unknown or variable minimum cut-angle. Ideally, in order to provide a smooth current dimming profile, where the minimum cut angle of the dimmer is mapped onto the full on (undimmed) current state of the ballast, the flicker in the undimmed state would vary between installations using different phase-cut dimmers. Consequently, in order to facilitate adaptive dimming, whereby the dimming range of the ballast can be mapped onto the phase-cutting range of the dimmer, the ballast should ideally include a mechanism by which current ripple is suppressed, or sufficiently reduced, at all values of cut-angle ϕ, at least within the first quadrant.
For the purposes of dimming, and in view of the fact that the light output from an LED is proportional to the current drawn by the LED, it is desirable to introduce a mechanism by which the current through the LED load is well-defined for any given cut angle, and where the said current is reduced (dimmed) in response to increasing cut angle. This may involve a control circuit operative wherein the cut-angle is translated into a PWM signal that is triggered by, and therefore at the same frequency as, the full-wave rectified input voltage.
Such a scheme is known in the prior art, as exemplified by Chu et al (U.S. Pat. No. 8,193,738). This discloses an LED power supply, or ballast, which, through the use of a modulated current control unit, together with a forward secondary winding on a mains transformer, both reduces the ripple current for a given smoothing capacitance and enables the use of larger smoothing capacitors. Whilst this, and similar schemes, represent a significant improvement in ripple performance and therefore flicker performance, the remaining ripple would still produce appreciable flicker, detectable by a significant proportion of observers.
The fundamental shortcoming of such approaches is that they continue to rely, to a large extent, on the smoothing effect of a capacitor as the main mechanism by which ripple and therefore flicker is reduced. This in particular means that whilst the disclosure of Chu et al enables the value of the smoothing capacitance to be increased, it retains the reliability problems experienced by power supplies that use high values of output capacitance. Furthermore, increasing the output capacitance for the purposes of reducing ripple and flicker means increasing the physical size of the ballast.
A more generalised scheme for translating phase-cut information from a phase-dimmer, into a controllable current dimming mechanism, whereby the degree of current dimming has a 1:1 relationship with the cut-angle, ϕ, is disclosed in aspects of exemplary embodiments of Lys et al (U.S. Pat. No. 7,038,399). Here again, however, any suppression of current ripple at the second harmonic of the mains frequency and therefore photometric flicker at this second harmonic, relies substantially on the action of a parallel combination of the differential impedance of an LED load and a smoothing capacitor.
A second limitation of both U.S. Pat. No. 8,193,738 and U.S. Pat. No. 7,038,399 lies in the fact that as the current extracted from a switch-mode power supply is reduced, to enact deep-dimming, there comes a point at which the current and therefore power demanded by the load is less than the minimum power deliverable by the power supply in continuous switching (i.e. non-hiccupping) mode. If dimming were attempted beyond this point, the power-supply would typically go into ‘hiccup mode’ whereby it delivers short bursts of charge to its output capacitor, sufficient to keep the capacitor charged, thereby maintaining an open-circuit output voltage that varies in a pulse-like fashion. This pulse-wise voltage would, if the power supply were dimmed so deeply as to bring about hiccup mode, then appear across the load, comprising at least in part an LED or collection of LEDs, thereby generating photometric flicker at the repetition frequency of the hiccupping. The output power of the power supply at which hiccup mode would be entered, therefore defines the minimum output power under normal operation and therefore the maximum dimming depth of the overall lighting ballast incorporating the power supply.
A limitation that applies to most dimmable switch-mode LED lighting ballasts, lies in the fact that as the output current and therefore output power, are dimmed, there comes a point below which the efficiency of the power supply falls significantly—by e.g. more than 10 percentage points below its undimmed value. This means that the oscillatory power, taken from the input rectifier does not fall pro-rata with the output power, delivered to the LED load. This in turn, results in increased percentage voltage ripple across the output capacitor of the ballast, and therefore increased percentage current ripple in the LED load. This translates to increased optical flicker percentage in the LED load. Furthermore, any such fall in efficiency during dimming will result in input power ceasing to fall pro-rata with output power, thereby militating the effect of dimming. Therefore, introducing a means by which efficiency can be more substantially maintained throughout the dimming process ensures that input power is more significantly reduced during dimming, thereby increasing energy saving arising as a result of dimming.
There therefore exists in the art, both from a photometric flicker perspective and from the perspective of maximising LED lifetime, a need to reduce more significantly, and preferably to a level of around 2% peak-to-peak or lower, the current ripple emanating from an LED ballast which in turn, is taking a phase-cut AC input, for the purpose of actuating dimming. Also, for the purposes of maximising the energy-saving potential of LED lighting, such a low flicker ballast should possess a wide dimming range, preferably from a full-on current down to less than 0.1% of the full-on current. Furthermore, the current ripple should ideally remain below 2% peak-to-peak throughout the entire dimming range.